Ask A Scientist© Weather Archive

name        Laura G.
status      other
age         old

Question -  Does dew rise from the ground or does it fall?
-------------------------
Laura,

Dew neither rises nor falls. Dew is simply airborne water vapor that has
condensed on cool objects. Since soil may contain more water than the
air several feet above. As the grass cools off, nearby soil moisture
condenses on it. Even so, wherever moist (humid) air has access to a
cold surface, condensation will occur. It will "dew" it every time.


Regards,
ProfHoff
=========================================================
Dew or frost forms on objects that have cooled
to less than the dew point or frost point (the point at
which air is saturated with water), respectively.

Objects near the ground will cool more rapidly at
night than the air itself, which is somewhat transparent
to terrestrial radiation (heat energy leaving the ground).
So dew or frost will normally occur before a fog will form.

The supply of water for dew or frost normally comes from
water in the air, although excess water in the air can come
from the soil and/or in plant leaves, particularly
after a day of rain. Evaporation of water from the ground
continues even at night and results in a shallow, more
humid layer of air near the ground, resulting in some
soaking dews on grass at times.

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
========================================================
Laura,

Since the source of the water is the aurrounding air,
I would say it 'falls'.  What actually happens is that
the air containing a certain percentage of water vapor
(expressed as a percentage of what it is able to hold
at that temperature--namely, the air's relative
humidity) cools and reaches a 'dew-point' temperature
at which the relative humidity becomes 100%.  This
means that because cooler air can hold less water, the
lower air temperature means that as the temperature
falls further, the water vapor will condense from the
air.  Since the air closest to the ground, and the
grass on it, is relatively cooler than the air above
it, the condensation occurs there.  As the temperature
cools further, more and more water vapor will condense
out.  Note that in the morning if the sun begins to
heat the surrounding air again, it can again hold
relatively more water vapor and the dew will
re-vaporize and be held in the air.

Thanks for using NEWTON!

Ric Rupnik
=========================================================
Dear Laura-

   Dew forms when the air temperature cools enough to cause the humidity in
the air to condense.
The dew comes from the air, and condenses on objects on the ground. The
moisture in the air may
have originally come from the ground, but it evaporated into a gas, which
later may condense as
dew.

   You can visit this web site for a more formal explanation of dew, the dew
point temperature, and
how dew forms.

     http://lawr.ucdavis.edu/coopextn/frostprotection/fp003.htm
Wendell Bechtold
=========================================================